Well, it sounds like things are on track then, do you have a tds meter for your RODI? There may be high impurities going out of the RODI...the chemipure is a good product...the large water changes will help as well.
I remember a thread here on r2r about hair algae problems using the Red Sea pro salt, however, I am not sure how it played out...might be worth a search...
the reduced feelings should also help....are you rinsing the food after it thaws in RODI? Might also help if you are not...
Another suggestion is to use Kent Tech M or fauna Marin algae X as a help ( I have used the algae x before and had good results) you may also be fighting a phosphate problem due to the rocks leaching it out...
do you have a refugium as part of the sump with the live rock and sanded? If so, do you have macro algae in it already? Your sand bed if not deep may also be harboring detritus ... If it is a 4-5" sand bed be careful and don't vacuum it. If it's shallow maybe remove it to eliminate possibilities.
Only other thing I can think of is if you are using any filter floss / pads / socks....might be creating a nitrate sink if not changed often enough...hope this helps..
One word of suggestion though *** only change 1 variable at a time ( with the exception of filter floss or socks) too many changes will make it hard to pinpoint the source of the problem and never seem to make fish or corals happy.